1. Field of the Invention
The present invention relates generally to light emitting devices and more particularly to phosphor converted light emitting devices.
2. Description of the Related Art
The emission spectrum of a light emitting diode (LED) typically exhibits a single rather narrow peak at a wavelength (peak wavelength) determined by the structure of the light emitting diode and the composition of the materials from which it is constructed. For example, the emission spectra of AlxInyGazN based LEDs typically peak at wavelengths from about 400 nanometers (nm) to about 590 nm and typically have full widths at half maximum of about 20 nm to about 50 nm. Similarly, the emission spectra of AlxInyGazP based LEDs typically peak at wavelengths from about 580 nm to about 660 nm and typically have full widths at half maximum of about 13 nm to about 30 nm. In the notations AlxInyGazN and AlxInyGazP, it should be noted that 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and x+y+z=1.
The most efficient LEDs emitting at wavelengths between about 400 nm and about 660 nm (as characterized by external quantum efficiency, for example) are currently AlxInyGazN and AlxInyGazP based LEDs. However, the efficiency of an LED of either type is strongly dependent on the peak emission wavelength of the LED. In particular, the efficiencies of AlxInyGazN and AlxInyGazP based LEDs show a broad minimum at peak emission wavelengths between about 515 nm and about 590 nm. This broad minimum, which unfortunately includes the spectral range in which the sensitivity of the human eye peaks, inhibits the development of many otherwise commercially attractive applications for LEDs.
An additional drawback of conventional AlxInyGazN based LEDs is the substantial blue shift in their emission which occurs with increasing drive current, particularly for LEDs having peak emission wavelengths between about 510 nm and about 590 nm. This blue shift is very noticeable for drive current modulation methods used to regulate LED radiance in display applications, for example, and consequently must be compensated for at additional cost.
A further drawback of conventional AlxInyGazN and AlxInyGazP based LEDs is the variability in emission characteristics between LEDs from separate wafers, or from different regions on the same wafer, prepared under nominally identical manufacturing conditions and by nominally identical methods. This variability results from the sensitivity of an LED""s emission characteristics to small variations in, for example, the composition and thickness of the various semiconductor layers it includes, particularly those in the active region. As a result of this variability, it is difficult to achieve high production yields in narrow peak wavelength ranges (bins).
What is needed is an LED based light emitting device that overcomes these and other drawbacks of conventional LEDs.
A method of fabricating a light emitting device includes providing a light emitting diode that emits primary light, and locating proximate to the light emitting diode a (Sr1xe2x88x92uxe2x88x92vxe2x88x92xMguCavBax)(Ga2xe2x88x92yxe2x88x92zAlyInzS4):Eu2+ phosphor material capable of absorbing at least a portion of the primary light and emitting secondary light having a wavelength longer than a wavelength of the primary light. The composition of the phosphor material, i.e., the values of u, v, x, y, and z, can be selected to determine the wavelengths of the secondary light. In one implementation the light emitting diode includes an AlxInyGazN material. In another implementation, the light emitting diode is a laser diode.
In one embodiment, the light emitting device includes the (Sr1xe2x88x92uxe2x88x92vxe2x88x92xMguCavBax)(Ga2xe2x88x92yxe2x88x92zAlyInzS4):Eu2+ phosphor material dispersed as phosphor particles in another material disposed around the light emitting diode. The phosphor particles and the material in which they are dispersed may be disposed as a layer on one or more surfaces of the light emitting diode. The material in which the phosphor particles are dispersed is selected, for example, from materials including but not limited to epoxies, acrylic polymers, polycarbonates, silicone polymers, optical glasses, and chalcogenide glasses. Preferably, the material in which the phosphor particles are dispersed has a refractive index greater than about 1.5. More preferably, the refractive index is greater than about 2.1. The concentration of the phosphor particles can be selected to absorb a predetermined fraction of the primary light and thereby adjust the chromaticity of a mixture of the primary and secondary light.
In another embodiment, the light emitting device includes the (Sr1xe2x88x92uxe2x88x92vxe2x88x92xMguCavBax)(Ga2xe2x88x92yxe2x88x92zAlyInzS4):Eu2+ phosphor material deposited as an optically homogeneous phosphor film on one or more surfaces of the light emitting diode. The phosphor film may be, for example, a film of substantially pure (Sr1xe2x88x92uxe2x88x92vxe2x88x92xMguCavBax)(Ga2xe2x88x92yxe2x88x92zAlyInzS4):Eu2+ phosphor material. The thickness of the phosphor film can be selected to absorb a predetermined fraction of the primary light and thereby adjust the chromaticity of a mixture of the primary and secondary light.
Light emitting devices in accordance with embodiments of the present invention are efficient light sources in the spectral range in which conventional AlxInyGazN and AlxInyGazP based LEDs show a broad minimum in efficiency, exhibit little or no blue shift with increasing drive current, and may be manufactured with high production yields in narrow peak wavelength ranges.